Torsional Vibration Dampers (TVDs) are useful in attenuating torsional vibrations inherent to rotating shafts, including but not limited to crank-shafts, drive-shafts, prop-shafts, and half-shafts utilized in automotive and non-automotive applications.
Commonly, a TVD consists of three components: (1) a rigid metallic bracket (hub) attaching the TVD to the rotating shaft with the vibratory problem; (2) an active inertial member (ring) oscillating opposite in phase to the vibrating shaft at a particular frequency, thereby reducing the resulting magnitude of the shaft vibration; and (3) an elastomeric member with two functions: (a) providing a spring damper thereby tuning the TVD to a particular frequency, and (b) locating the hub and the ring with respect to each other in the TVD.
Commonly, the hub and ring are metallic in construction due to the structural strength requirement for the hub and the inertial mass requirement for the ring (also known as an inertia member). The elastomeric member is first compression molded as a strip followed by assembly under compression between the hub and ring where it assumes an axis-symmetric ring like shape. At times the surfaces of the hub and ring that mate with the strip (profile) are coated with a primer-adhesive combination that causes the strip to be bonded to the aforementioned surfaces. The disclosed invention pertains to both bonded and non-bonded dampers as long as they employ a compression molded elastomer strip.
The elastomer used in the elastomer member could be one of several thermoset material alternatives, including but not limited to styrene butadiene rubber (SBR), poly butadiene (PBD); ethylene propylene diene monomer (EPDM); nitrile butadiene rubber (NBR), or any possible combination thereof. Furthermore, the elastomer could be one of several thermoplastic material alternatives, including but not limited to styrenic block copolymers (TPE-s), polyolefin blends (TPE-o), elastomeric alloys (TPE-v or TPV), thermoplastic polyurethanes (TPU), thermoplastic copolyester, and thermoplastic polyamides, or any possible combination thereof.
The main purpose for utilizing a TVD is to extend the fatigue life of the vibrating shaft by reducing the resulting amplitude at a particular frequency where the inertia ring counteracts the shaft vibration by oscillating with an enhanced magnitude but opposite in phase with the shaft vibration (vibratory influence). However, in many crankshaft applications, the ring has poly-vee grooves for driving the serpentine belt of a front end accessory drive (FEAD) system, which may include, but is not limited to, an alternator, water-pump, fan, tensioners, and idler pulleys (load-bearing influence).
Due to the aforementioned loading scenarios, the elastomer members used in TVDs undergo two separate modes of loading: (1) normal loading caused by assembling the elastomer member under compression between the hub and ring; and (2) shear loading caused by the operation of the TVD both from vibratory and load-bearing influences.
To regulate the normal stress-strain bearing capacity of the elastomer member, the compression of the elastomer member and the profile geometry is designed to minimize the maximum principal stress and maximum principal strain buildup in the elastomer member post assembly.
To regulate the shear stress bearing capacity of the elastomer member, a threshold parameter called the slip-torque of the damper is established. The slip-torque is defined as the minimum value of quasi-static torque that causes the permanent angular slip across any elastomer metallic interface (either between the elastomer member and ring or between the elastomer member and hub).
It can be appreciated that the proper engineering of an elastomer member is a balance between maximizing the slip-torque capacity of the TVD (directly proportional to the compression of the elastomer member) while minimizing the principal stress-strain buildup (inversely proportional to the compression of the elastomer member). In the industry, a compression ratio of 30% to 40% is currently accepted as the norm where such a balance is obtained.
Furthermore, there is a constant effort by elastomer member manufacturers to increase production yield by simultaneously increasing the number of elastomer members produced per molding heat, while decreasing the time required for each molding heat.
This objective retains its value post-molding where secondary operations are avoided when possible. One such secondary operation relates to the removal of the flimsy residual elastomeric membrane (flash) present on the surface of the elastomer member visible post-assembly. Flash removal may be accomplished by grinding, cutting, cryogenically tumbling the elastomer members (de-flashing), or other methods known to one of skill in the art. This aforementioned flash is a byproduct of the compression molding operation, and is unsightly and undesirable as it could contaminate the hub nose region that interfaces with the front engine seal, causing leaks and failures therein.